12 October 2017
By Michael Le Page
We’ve found out why a Mexican cavefish has no
eyes – and the surprising answer is likely to be seized upon by those who think
the standard view of evolution needs revising.
Over the past few million years, blind forms
of the Mexican tetra (Astyanax mexicanus) have evolved in caves. Maintaining eyes and the visual parts
of the brain uses lots of energy, so the loss of eyes is a big advantage for
animals living in the dark. Instead the cavefish “see” by sucking.
It was assumed that these fish became blind
because mutations disabled key genes involved in eye development. This has been
shown to be the case for some other underground species that have lost their
eyes.
But Aniket Gore of the
US’s National Institute of Child Health and Human Development and colleagues
haven’t found any disabling changes in the DNA sequences of eye development
genes in the cavefish.
Instead, the genes have been switched off by
the addition of chemical tags called methyl groups. This is what is known as
an epigenetic, rather
than genetic, change.
“Although a central role for DNA methylation
in development and disease has been well-documented, our results suggest that
epigenetic processes can play an equally important role in adaptive evolution,”
the team writes.
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